There has always been a need to separate out target signals from background noise, whether the signals in question are sound or electromagnetic radiation. In the field of sound, noisy environments such as in modes of transport and offices present a communications problem, particularly when one is attempting to carry on a phone conversation. One known approach to this problem is a two-microphone system, wherein two microphones are placed at fixed locations within the room or vehicle and are connected to a signal processing device. The speaker is assumed to be static during the entire use of this device. The goal is to enhance the target signal by filtering out noise based on the two-channel recording with two microphones.
The literature contains several approaches to the noise filter problem. Most of the known results use a single microphone solution, such as is disclosed in S. V. Vaseghi, Advanced Digital Signal Processing and Noise Reduction, John Wiley & Sons, 2nd Edition, 2000. In particular, the single channel optimal solution (optimal with respect to the estimation variance) was disclosed in Y. Ephraim and D. Malah, Speech enhancement using a minimum mean-square error short-time spectral amplitude estimator, IEEE Trans. on Acoustics, Speech, and Signal Processing, 32(6):1109-1121, 1984. A modified variant of that estimator was disclosed in Y. Ephraim and D. Malah, Speech enhancement using a minimum mean-square error log-spectral amplitude estimator, IEEE Trans. on Acoustics, Speech, and Signal Processing, 33(2):443-445, 1985, the disclosures of all three of which are incorporated by reference herein in their entirety.